The purpose of the proposed research is to make new measurements of fundamental elastic and contractile properties of cardiac muscle in order to better understand the interaction of these properties during cardiac muscle contraction. Although the development of the muscle twitch traditionally has been explained in terms of active contractile elements delivering force through passive elastic elements, quantitative models based on such reasoning have shown serious limitations. Alternative methods for studying heart muscle have recently been developed which involve rapid controlled alterations of muscle length instead of previous methods of controlling force and measuring resulting length changes. Important discrepancies have been found suggesting that muscle elasticity may vary substantially during a twitch, thus both contradicting the traditional passive description of muscle elasticity, and introducing major errors into previous methods of evaluating contractile properties. The present research will measure rapid muscle force and length changes with new high-resolution equipment by a much more detailed and systematic procedure than previously. Experimental parameters such as amplitude, duration, velocity, time in the twitch, previous mechanical history, and choice of force or length as independent variable will be controlled and systematically varied. Results will be interpreted in terms of possible active and passive elastic, contractile, and viscoelastic structures of the muscle cell. It is hoped that the fundamental knowledge thus obtained will lead to an improved understanding of underlying events during normal and abnormal contractions of the heart.